Bugfix/non ascii in xml (reorganized)

Yet another PR for non-ASCII and XML related issues #787 and #628 (reorganized version of #796 ).

The previous PR #796 is some shortcut approach to the bugs, so
this PR reorganizes the diffs into smaller/step-by-step pieces.
(The total diffs of these PRs are identical.)

The reviewer(s) can choose from some options:
- Review #796 only, without looking at this. (yes, they are identical :) )
- Approve this PR (this or #796).
- Approve only first two commits (449d099 and c7eed7c). #628 will not be fixed in this case.
- Decline both. :P

Additional comments (which is not proper for commit messages), in chronological order:

- 449d099 * Unicode-aware handling of non-ASCII object keys in multipart uploads

  Direct/honest/strait fix of #787

- c7eed7c * Add tests of multipart upload with non-ASCII objet key and listing uploads

  Boto tests for the previous commit 449d099

- 48be327 * Add Unicode handling of lists in the same manner as binaries

  As commit message says. :)

- 84a821c * Generarize riak_cs_xml:to_xml/1 to convert xmerl's simple form

  To utilize riak_cs_xml's automatic conversion logic, extends `riak_cs_xml:to_xml/1`
  to accept xmerl's simple forms and generate XMLs.

- d35d74a * Use XML export of simple forms of riak_cs_xml:to_xml in multipart uploads

  Use the to_xml in previous commit 84a821c. For multipart related logic.
  This commit overwrites the changes in the first commit 449d099.

- d215335 * Use XML export of simple forms of riak_cs_xml:to_xml instead of export_xml/1

  Same as previous one, but other than multipart related ones.

- 9f1d2cd * Fix bug of accept content type

  Not related to non-ASCII bug. Just a bug fix of test case.

- 863a87a * Add test case of listing users which has non-ASCII UTF-8 characters

  As commit message says.

- d78cb66 * origin/bugfix/non-ascii-in-xml-reorganized bugfix/non-ascii-in-xml-reorganized Add test case of access stats with xml format

  As commit message says.